These Unggoy are used to engage high-tier enemies through the use of augmented armor, explosives such as the Type-33 fuel rod gun and Type-58 fuel rod cannon.[4][5] This class is composed of experienced and formidable Unggoy warriors, who are often fully capable of engaging light armed vehicles effectively. Unggoy of this role are rarely encountered on the battlefield though.[6]

The original Unggoy Heavies wore distinctive sage-green armor, typically with a full-face methane breather and a cylindrical methane tank on their back, similar to the Unggoy Ultra. Their armor provided significantly more protection that that of an Unggoy Minor or Major but it wasn't nearly as strong as an Ultra's or a Special Operations Unggoys in terms of durability.[8]

The Heavies of Jul 'Mdama's Covenant wear a helmet with full-face protective rebreathers, enhanced optical sensors with optics articulators, and ultra-dense battle harnesses that has a slim-stock tank with analog eye sockets of Unggoy predators. The harness is issued in orange, blue, silver, green, and magenta.[6] The combat harness also allows for the Heavies to use the methane tank as a makeshift thruster pack, allowing for easy access in various terrains.[4][5]

In Halo 3, if a Heavy panics while carrying an undeployed plasma turret, it will wave the turret around in the air with only one hand. This is because Unggoy usually use small single-handed weapons, and no separate animation was created for Unggoy Heavies using turrets.

Unggoy Heavies are classified as Special Operations Unggoy in Firefight, whereas Unggoy Ultras are simply classed as regular Unggoy. Actual Special Operations Unggoy are only seen manning the side-mounted plasma turrets on the Phantoms. Something similar happens in Halo 4, where Unggoy Heavies are classified as Ultras by the Commendations along with some other enemies who have also been mislabeled.